In vehicles having an internal combustion engine with an emission control device, NOx traps are often used to decrease the amount of NOx released in the exhaust. The amount of NOx in the exhaust may be elevated when an engine is operating lean. Lean operation improves fuel economy by reducing the pumping losses and improving thermodynamic efficiency. However, the exhaust from such an engine may contain large amounts of excess oxygen for extended periods of time, and thus increase the amount of NOx in the emissions system. In other circumstances, feedgas NOx generation can decrease in lean operation, depending on the operating conditions. These conditions of extended lean operation can affect NOx conversion of three way catalysts.
To control the NOx emissions during lean operation emission control devices capable of storing NOx during lean operating conditions (lean NOx traps) can be used. Periodically, as the NOx capacity of the NOx trap is approached, the air-fuel ratio (A/F ratio) can be driven to a rich condition for a few seconds in order to purge the trap of stored NOx and to regenerate the NOx storage capacity of the trap.
While lean NOx traps can provide high conversion of NOx under such lean/rich cycling conditions, the catalyst system should also provide high conversion of HC, CO, and NOx when the A/F ratio is controlled about stoichiometry, for example during high load operation. If the close-coupled catalysts contain low amounts of oxygen storage capacity (OSC), the ability of these catalysts to convert CO and NOx under the oscillatory A/F conditions characteristic of closed-loop control systems may be limited. Therefore, the NOx trap itself may contain some OSC in order to provide high CO and NOx conversion under these conditions.
The inventors herein have recognized a disadvantage of including cerium in a lean NOx trap to provide OSC. Cerium may cause some of the stored NOx to be released from the trap during the purges without being reduced to N2. This purge NOx release, without conversion, may be particularly evident at temperatures of 350xc2x0 C. and above, and may increase as the amount of stored NOx on the lean NOx trap is increased. A major source of this purge NOx release may be attributed to the exotherm that results from the reaction between the reductants in the exhaust (HC, CO, and H2) and oxygen from the cerium. This exotherm heats up the local area of the washcoat around the cerium site, including any NOx storage sites nearby. Since the oxygen from the cerium has consumed the reductants, the released NOx may not be reduced, but rather can be emitted from the trap into the exhaust and out the tailpipe. Another possibility is that the oxygen released from the ceria competes with the released NOx for the reductants. As a result of this competition, some of the NOx may not be reduced but can be emitted from the tailpipe.
Thus, the inventors herein have recognized that a disadvantage of emission control systems whose rich purging is based on lean NOx trap storage efficiency or storage capacity is that the amount of NOx released in tail pipe emission may still be too high, and specifically, that this purged NOx may be at least partially attributed to the element providing OSC within the NOx trap.
The above disadvantage can be overcome by a method for controlling an engine having an exhaust with an emission control device capable of storing NOx during lean operating conditions, and at least partially converting said NOx during stoichiometric or rich operating conditions, comprising:
operating the engine to produce a lean exhaust gas mixture fed to the emission control device;
calculating an amount of NOx stored in the device based on operating conditions;
determining a NOx storage limit value, said value determined as a function of device temperature and an amount of oxygen storage capacity of the device; and
ending said engine operation producing said lean exhaust gas mixture when said amount of NOx stored reaches said limit value.
Thus, the inventors herein have recognized that the NOx emitted from a NOx trap during rich purging may be further minimized by limiting the amount of NOx stored during lean operating conditions to a level that is a function of the trap temperature and the trap OSC. Note that the limit value may be a lower amount than the maximum storage capacity or an amount calculated to satisfy a predetermined lean storage efficiency. Purging the trap when it reaches this limit of stored NOx can thus minimize the emitted NOx during purges.
In another aspect, the above disadvantage can be overcome by a computer storage medium having instructions encoded therein for controlling an engine having an exhaust with an emission control device capable of storing NOx during lean operating conditions, and releasing and at least partially converting said NOx during stoichiometric or rich operating conditions, said medium comprising:
code for operating the engine to produce a lean exhaust gas mixture fed to the emission control device;
code for calculating an amount of NOx stored in the device based on operating conditions;
code for determining a NOx storage limit value, said value determined as a function of device temperature and an amount of oxygen storage capacity of the device; and
code for ending said engine operation producing said lean exhaust gas mixture when said amount of NOx stored reaches said limit value.
By utilizing these encoded instructions, it is possible to minimize the purge NOx release that is not converted by purging the NOx trap when the amount of stored NOx reaches a limit value determined by the trap""s temperature and OSC. Also note that at least some NOx is converted, in some cases, it is desirable to attempt to convert and much NOx as possible.
Further, the inventors herein have also recognized that several factors influence the amount of purge NOx release from the NOx trap, including the amount of NOx stored, the OSC of the trap, the trap temperature, and the amount of reductant supplied during the purge. All these factors can be considered in the effort to reduce the amount of emitted NOx. Thus, the inventors have discerned that NOx trap purging can be improved when the engine operates to produce a rich air-fuel exhaust ratio selected to provide a level of reductant dependent on the trap temperature and the trap OSC.